The present invention is directed to a method and arrangement for inverting the states of the cells of a plasma panel, or similar display device, on an individual, addressable basis.
In its most conventional form, a plasma panel is comprised of two dielectric plates, or surfaces, at least one of which is translucent, between which a continuous body of gaseous display material such as neon, is sealed. A first set of "column" conductors is disposed on one plate in a generally vertical direction. A second set of "row" conductors is disposed on the other plate in a generally horizontal direction.
The individual regions of the panel defined by the intersections, or crosspoints, of the various row and column conductors function at its display cells. Pictures, text and other graphical data are presented on the panel by creating individual glow discharges in the gas at selected crosspoints under the control, for example, of a digital computer. The computer initiates a discharge at a particular cell by impressing, or applying, a "write" pulse thereacross via its row and column conductor pair. The magnitude of the write pulse exceeds the breakdown voltage of the gas, and a space charge, or plasma, of electrons and positive ions is created in the crosspoint region. Concomitant avalanche multiplication creates a glow discharge and accompanying short, e.g., one microsecond, light pulse in the visible spectrum. The write pulse, which continues to be applied to the cell, begins to pull the space charge electrons and ions, or charge carriers, to opposite cell walls, i.e., the opposing dielectric surfaces in the crosspoint region. When the write pulse terminates, a "wall" voltage resulting from these so-called wall charges remains stored across the gas at the crosspoint.
A single short-duration light pulse cannot, of course, be detected by the human eye. Thus, in order to provide a plasma display cell with the appearance of being continuously light-emitting (ON, energized), further rapidly successive glow discharges and accompanying light pulses are needed. These are generated by a "sustain" signal which is impressed across each cell of the panel. The sustain signal may comprise, for example, a train of alternating-polarity pulses. The magnitude of these sustain pulses is less than the breakdown voltage. Thus, the voltage across cells not previously energized by a write pulse is insufficient to cause a discharge and those cells remain in a non-light-emitting state.
The voltage across the gas of an ON, i.e., previously-energized, cell, however, comprises the superposition of the sustain voltage with the wall voltage previously stored at that cell. The sustain pulse which follows a write pulse has a polarity opposite thereto. As a result, the wall and sustain voltages combine additively across the gas. The combined voltage may be assumed to exceed the breakdown voltage. Thus, a second glow discharge and accompanying light pulse are created. The flow of carriers to the cell walls now establishes an opposite wall voltage polarity. The polarity of the next sustain pulse is also opposite to that of its predecessor, creating yet another discharge, and so forth. After several sustain cycles, the magnitude of the wall voltage reaches a constant, characteristic level which is a function of the gas composition, panel dimensions, sustain voltage level, and other parameters. The sustain signal frequency may be on the order of 50 kHz. Thus, the light pulses emitted by an ON cell in response to the sustain signal are fused by the eye of the viewer, and the cell appears to be continuously energized.
A cell which has been established in a light-emitting state is switched to a non-light-emitting (OFF, de-energized) state by removing its wall charge. In particular, an "erase" pulse is applied to the cell in question, again via its row and column conductor pair. The erase pulse polarity is opposite to that of the preceding sustain pulse and, although its magnitude is typically somewhat less than that of a sustain pulse, it is of sufficient magnitude to cause a discharge at an ON cell. Thus, the wall voltage begins to reverse polarity. However, the erase pulse is of such short duration relative to a sustain pulse that the wall voltage reversal is terminated prematurely, at a time when the wall voltage magnitude is less than the difference between the breakdown and sustain voltages. Thus, no further breakdowns occur and the cell is returned to an OFF state.
It will be appreciated from the foregoing that a plasma panel has what is referred to as "inherent memory"; once the computer or other controlling apparatus applies a write pulse to a selected cell, the cell remains in an energized state with no further computer intervention. The latter is thus freed for other tasks until some change in the displayed image is to be made. By contrast, a cathode-ray tube (CRT) display, for example, does not have inherent memory; the phosphor regions thereof emit light only in response to an applied electron beam and once the beam passes by a particular region on its scan across the display, the beam leaves behind no evidence that a particular region had been energized. Thus, CRT systems require a separate "frame memory" to store a representation of the image being displayed. The controlling apparatus must continually refer to this memory (typically 30 times per second) to refresh the display, even when the displayed image is not to be changed.
The fact that a typical plasma display system does not have a frame memory is a mixed blessing, however. As is illustrated in the Detailed Description hereof, a number of display applications require, or at least would benefit from, the ability to invert (flip, toggle) the states of one or more cells on a selectable, addressable basis-- that is, the ability to change a cell from the particular one of its (illustratively) two states in which it resides to the other of its states. Cell-state inversion is easily accomplished in a display system having a frame memory since the controlling computer, for example, has a record of the state of each display cell and can generate appropriate signals on the basis thereof to modify the displayed image as desired.
By contrast, the computer controlling a plasma panel typically has no such record to which it can refer. Thus, it cannot readily determine whether a write or an erase pulse is needed to invert the state of a particular cell. One solution is to augment the system with the otherwise redundant and unnecessary frame memory. Another is to "read out" (in known manner) the state of the cell to be inverted. Disadvantageously, however, both of these alternatives add appreciably to the cost and complexity of the display system.